This invention relates to the application of chrome finishes to plastic articles and, more particularly, to an improved vacuum metallizing process for applying chromium coatings on rigid and flexible plastic articles.
Plastics offer a lower cost, lighter weight substitute to metals and alloys as a material of construction in a wide variety of articles of manufacture. In the automotive industry, for example, more emphasis than ever is being placed on the use of plastics in place of metals both to lower the cost of manufacture and to reduce vehicle weight to thereby increase the fuel efficiency of the vehicles. Examples of parts where this substitution can be made include head and rear lamp bezels, integral signal lamp housings and bezels, side marker lamp housings, radiator grills, bumper components, window trim, and wheel covers. However, when plastics are substituted for metals, it is often highly desired that the parts be provided with a bright, reflective surface simulating the metallic lustre of the parts they replaced. In automotive applications, it is specifically desired that certain plastic parts be provided with a bright, reflective metallic surface simulating the traditional appearance of chrome-plated steel.
Vacuum metallizing offers a relatively low cost means for obtaining metallic finishes on plastic parts. Vacuum metallizing refers generally to a process wherein the coating metal is evaporated by resistance heating and is then condensed on the surface of the article to be coated all at a subatmospheric pressure to prevent the oxidation of the coating metal during the evaporation and deposition steps. Conventional resistance-fired vacuum metallizing processes employ a large number of metal heating filaments, typically tungsten in a spiralled configuration, spaced along the length of a rack upon which the articles to be coated are mounted. The metal to be evaporated and deposited on the articles is in the form of small wire strips which are suspended on the individual tungsten heating elements. When an electric current is passed through the tungsten elements, the elements rise in temperature thereby melting the coating metal which then flows over the heating elements and adheres thereto. On increasing the current through the heating elements, the liquid metal is evaporated and subsequently condenses on the articles to be coated. The entire process is carried out in a vacuum in the range of about 10.sup.-3 to 5.times.10.sup.-4 mm Hg. Aluminum and other metals and alloys have been vacuum metallized by such conventional resistance-fired processes in the manufacture of parts for interior applications in the automotive industry. Although such coatings have been successful in interior applications, they have proved unsuccessful in exterior applications where the corrosion and abrasion resistance of the coatings become important. For example, resistance-fired aluminum has proved unsuccessful in exterior applications because of its inherent tendency to corrode to a soft, dull aluminum oxide. For vacuum metallizing to be an acceptible method of coating plastic articles for exterior applications in the automotive industry, it must provide a metal finish possessing good color and brightness, corrosion and abrasion resistance sufficient to retain its bright finish for the life of the vehicle, good weatherability, and good resistance to road chemicals, gasoline, anti-freeze and car wash detergents.
Prior art attempts to provide coatings meeting these criteria have employed more highly corrosion resistant metals and alloys such as stainless steel, nickel-chromium alloy, and chromium in conventional resistance-fired vacuum metallizing processes. Parts so coated have been found to be unacceptible for exterior automotive applications for a number of reasons. For example, chromium applied by conventional resistance-firing is much darker in color than electroplated chrome. It is often referred to as "black" chrome and does not have an appearance acceptible for exterior automotive applications. In substance, the means has not heretofore been reported for supplying rigid and flexible plastic parts with an adherent bright, reflective metal finish simulating the appealing traditional color and brightness of conventional electroplated chromium and possessing the requisite corrosion and abrasion resistance.